It is known in aerospace applications and elsewhere to provide a relay for controlling the ON and OFF states of a load connected to an alternating current supply with a relay, such relays being advantageous whenever the control command is at a low level and the level of electric current which should be controlled is significant and/or the load circuit must be isolated from the command circuit.
At the present time alternating current loads can be connected to or disconnected from a source of alternating current by either an electromagnetic relay, i.e. a relay having moving parts, or a static relay which is generally of solid-state construction and can be devoid of any moving parts.
Aerospace applications put control devices for such purposes to rigorous test, because the environment in which the control circuit must operate is one of mechanical shock represented by the high-energy forces of launching and acceleration or from collision, the stresses resulting from particle bombardment, and thermal extremes.
Whether these relays are of a mechanical or moving-part type or static type having no moving parts, they generally are designed to be bistable, i.e. to latch the load circuit in an operative condition upon receipt of an ON command and in turn deenergize the load upon receipt of an OFF command.
The mechanical relay can comprise a latching relay having two coils, one of which is energized to place the relay in the 0N state and the other of which can be energized to place the relay in the OFF state. The latching relay, therefore, is either on or off, depending upon which of the two coils is energized, and retains one of the two stable configurations until a configuration-change command arises at the other coil.
The latching relay systems currently in use for aerospace applications have both electrical and mechanical drawbacks.
Among the mechanical drawbacks are the delicacy of the latching relay mechanism which limits the acceleration to which the relay can be subjected and thus the application of the relay, high tendency to contact wear limiting the operational life in terms of the number of useful operations, and fragile internal structure which can be damaged beyond repair by mechanical shock.
The electrical drawbacks include the reliance for the electrical connection upon mechanical pressure provided by a spring urging the two contacts together, significant contact resistance differences from one unit to another and from time to time during use of each unit, thereby effecting detrimentally the reliability and reproducibility of the results. Furthermore, because of arcing phenomena, mostly with contact opening, contact surface deterioration occurs and radio frequency disturbances are created.
Latching relays have other disadvantages as well. For example, the time required for complete opening or closing is generally of the order of tens of milliseconds, thereby requiring command pulses longer than the minimum opening or closing durations to ensure complete execution of the command. When the relay is exposed to the command for periods on the order of milliseconds but less than the minimum required, a bounce effect may result with repeated openings and closings of the contacts.
It has thus been recognized that mechanical relays in many cases are not able to satisfy the desiderata of aerospace applications which require the relay to resist and tolerate the high vibrations and accelerations during a launching phase and the intensive particle bombardment of an orbit phase. Consequently, reliance has been placed largely upon static relays.
A static relay for control of alternating current generally comprises a triac or controlled rectifier which acts as a solid state static circuit-opening or circuit-closing element in the alternating current circuit.
The triac is, in turn, driven by a photo coupler which receives the external commands and provides the longer pulses required, while affording galvanic isolation of the command circuit from the alternating current load circuit.
Such prior art solid state relays also have been found to be inadequate for modern aerospace applications. For one thing, such solid state circuits use components, like photocouplers, which suffer an aging phenomenon accelerated by high doese of space radiation.
Furthermore, triacs are not generally among the components listed as acceptable for space applications under most standards because of poor reliability.